1. Field of the Invention
The present invention relates to a liquid ejection head that employs a scheme in which liquid is ejected by using thermal energy.
2. Description of the Related Art
Through similar processes to that for semiconductor manufacturing, a substrate for a liquid ejection head is manufactured by forming, on the same substrate, multiple heaters for heating liquid to generate bubbles when being energized, lines for providing electrical connection to the heaters, and the like. Then, a liquid ejection head is constructed in a way that a member (nozzle formation member) forming ejection openings and walls for liquid passages is provided on the substrate. Here, the ejection openings are provided corresponding to the heaters and are used to eject ink therefrom. Meanwhile, the liquid passages are formed to communicate with the corresponding ejection openings, respectively.
One method of manufacturing the liquid ejection head (see Japanese Patent Laid-Open No. H6-286149 (1994) includes the following steps:                (1) forming a pattern to form the liquid passages on the substrate with a dissolvable resin;        (2) applying a coating resin containing an epoxy resin being solid at ordinary temperature;        (3) forming openings to be the ejection openings in the coated resin; and        (4) dissolving the dissolvable resin layer.        
Further, there has been proposed a liquid ejection head and a method of manufacturing a liquid ejection head in which a layer made of a polyetheramide resin (called an adhesion improvement layer below) is interposed between the substrate and the nozzle formation member in order to improve the adhesion between them (see Japanese Patent Laid-Open No. H11-348290 (1999)).
As even higher printing fineness and higher printing speed are demanded of the liquid ejection head, an increased number of heaters are required to be implemented on the substrate. This largely increases the number of lines used for energizing the heaters. As a result, depending on the locations of the heaters, the lines extending from electrode terminals of the substrate to the heaters vary in length, and accordingly greatly vary in resistance value. A possible way of evening the resistance values of the respective lines is to determine a width of each of the lines according to the distance from the electrode terminal. In this case, however, the lines for heaters existing farther from the electrode terminals have larger widths, and therefore the substrate increases in size.
To suppress the increase in the substrate size, a configuration has been proposed in which a low-resistance line common to all the heaters is formed of a thick film on the substrate surface and in which an individual line is formed from the common line to each of the heaters (see Japanese Patent Laid-Open No. 2005-153499).
To further reduce the line resistance value, the following technique has been proposed. Specifically, the common line and the electrode portions are simultaneously formed as a gold (Au) layer by plating (see Japanese Patent Laid-Open No. 2005-199701). Gold has excellent properties as a line material because of its low electric resistance, high chemical stability, high electromigration characteristics, and the like. Particularly, gold is excellent as a line material of a substrate for a liquid ejection head because the lines ordinarily exist very close to the ink and are used to energize the heaters to raise their temperature instantly.
However, the present inventors have discovered that the following technical problems needing resolution arise if the configuration using a common line as described above, especially using gold as the common line, is applied to the liquid ejection head described in Japanese Patent Laid-Open No. H6-286149 (1994) or No. H11-348290 (1999).
In the configuration of the liquid ejection head described in Japanese Patent Laid-Open No. H6-286149 (1994) or No. H11-348290 (1999), metal surfaces of the lines and the like existing on the substrate adhere to an organic resin constructing the nozzle formation member or the adhesion improvement layer. This adhesion is thought to be brought by a physical anchor effect of the organic resin entering the dips in the metal surfaces, and also by chemical bond, hydrogen bond, or the like through the OH groups existing on the metal surfaces.
However, being a stable noble metal, gold has a few OH groups on its surface, and therefore has poor bonding power with an organic resin. In addition, on a liquid ejection head substrate, the organic resin film swells because ink constantly exists near the ejection openings. Particularly, in a liquid ejection head substrate with heaters, heat generated by the heaters causes the organic resin and the substrate to expand to different degrees. As a result, the liquid ejection head substrate with heaters undergoes internal stress caused by the difference in thermal expansion between the substrate and the organic resin, in addition to the swelling of the organic resin film. This stress could possibly cause separation of the nozzle formation member from the Au layer, originating from and around parts having poor adhesion with the organic resin.
Such separation causes electrolytic ink to invade into an interface between the organic resin layer and the gold (Au) lines. Then, such ink invasion causes the electrolysis of Au and the deformation of the nozzle formation member. As a result, sufficient reliability might not be obtained.
The problems given above are especially noticeable when gold is used as the lines, but are also concerned more or less when a metal other than gold is used.